The measurements of filling heights are conducted wherever the volumes of liquids and the alteration of volume have to be determined. The measurements of filling heights are usually done by electrodes, which immerse at least partially into the liquid. The electrical conductivity or the resistance of the liquid, which is proportional to the filling height or the volume of the liquid, is measured by a suitable measuring device.
Such measurements are necessary in order to determine the exhaustion of filter cartridges, which are used in gravitation driven filtration devices.
WO 02/27280 A discloses a device using three electrodes one of which is used as reference electrode. The electrodes for level measurement are configured in such a way that a measurement value sharply changes when certain limits of the level are exceeded or fallen short of. These leaps of the measurement values can be reliably recognized without high demands on the accuracy of measurement.
A similar device is known from EP 1 484 097 B1 which comprises at least three electrodes, counting means and timers. The signals measured by these components are fed to an input of a microprocessor that, on the basis of a resident programme, elaborates important data on the life-span of the cartridge according to the amount of time passed since its first activation and the amount of water treated identified in terms of closure considered important by circuit between the electrodes, and by the ionic concentration of the pollutants, identified in terms of conductivity of the water being treated.
In case of consideration a partial filling of the vessel numerous electrodes are located on growing levels in a compensation chamber within the vessel.
This device is expensive and never even takes into consideration the design and shape of the vessel. Exact measurements of volume require intermediate measurements of the filling height taking into account the vessel shape. Usually, the vessel has any design so that the correlation between filling height and liquid volume doesn't follow a simple mathematical formula.
Most of the measuring devices ignore vessel shape so that the determination of life-span of the filter cartridge is not as precise as it should be.
U.S. Pat. No. 4,724,705 A relates to a fuel measurement device and particularly a device for determining for quantity of a fuel in a fuel tank. The fuel level indicator includes a hollow housing, a coded wafer, a short circuit wafer including a wafer substrate, a buoyant member and a continuity bridge. The coded wafer is made of a dielectric, ceramic material and extends along the interior length of the hollow housing. An electrically conductive wire strand having a known resistance per unit length is wound about the coded wafer to define a “pattern of resistance” representative of the contour of the interior wall of the fuel tank. The manufacturing of the fuel level indicator is quite an effort, in particular for manufacturing of the coded wafer.
DE 10 2005 035 045 A1 discloses a measuring device comprising a measuring element that includes at least one electrode the area of which increases in an exponential manner from one and to the other. The benefit of this invention is the fact that the value of electrical conductivity and the absolute value of the liquid level in the vessel need not to be known, if there is an exponential correlation between the measuring values and the volume of the liquid in the vessel.